Stephanie is 4 times as old as William and is also 15 years older than William. How old is Stephanie?
Answer: We can use the given information to write down two equations that describe the ages of Stephanie and William. Let Stephanie's current age be $s$ and William's current age be $w$ $s = 4w$ $s = w + 15$ Now we have two independent equations, and we can solve for our two unknowns. One way to solve for $s$ is to solve the second equation for $w$ and substitute that value into the first equation. Solving our second equation for $w$ , we get: $w = s - 15$ . Substituting this into our first equation, we get the equation: $s = 4$ $(s - 15)$ which combines the information about $s$ from both of our original equations. Simplifying the right side of this equation, we get: $s = 4s - 60$ Solving for $s$ , we get: $3 s = 60$ $s = 20$.